On the evaluation of the target strength for comparison between monostatic simulations and measurements

The target strength (TS) quantifies the acoustic return on an active sonar from a submerged object. It is a far-field quantity that varies with frequency and orientation of the object with respect to the sonar source and receiver. Numerical modelling allows to perform simulations in the far-field at an arbitrary frequency. Experimentally, a sonar pulse covers a finite bandwidth, the presence of the water surface and other aspects of the environments complicate the propagation, and the presence of noise and reverberation limit the ability to measure the weakest components of the acoustic return. A numerical investigation is conducted to support the comparison and understanding of the differences between measurements and simulations in monostatic scenario (source and receiver in the same direction). Elastic spheres (for which an analytic solution is available) and an elongated object (simulated with Kirchhoff diffraction approximation) are considered. These objects are simulated in the far-field and in the near-field, at single frequencies and with pulses of several duration and bandwidth. Various TS metrics, including peak TS and integrated TS are evaluated. The comparison of these metrics with the single frequency TS is discussed, as well as the impact of measurement noise and reverberation on these metrics.